In the field of pressure-driven membrane separations (e.g., ultrafiltration, reverse osmosis, nanofiltration) there is frequently a problem of membrane fouling from contamination of other dissolved and suspended solids in feed streams. This kind of membrane separation has been used, for example, in apple juice clarification, waste water treatment, cheese whey desalting, potable water production, oil-water emulsion separation, etc.). This problem has been addressed in a variety of ways. For feed streams that are not fouling, hollow fiber membrane module are most efficient and cost effective means of separation. However, hollow fiber membrane designs will foul most easily and cannot be used for the majority of feed streams in industrial processing or waste treatment due to fouling problems.
The next most expensive membrane design in terms of providing the greatest membrane surface area in a vessel per cost is a spiral wound configuration. In a spiral wound configuration, a permeate spacer, a feed spacer and two membranes are wrapped around a perforated tube and glued in place. The membranes are wound between the feed spacer and the permeate spacer. Feed fluid is forced to flow longitudinally through the module through the feed spacer, and fluid passing through the membranes flows inward in a spiral through the permeate spacer to the center tube. To prevent feed fluid from entering the permeate spacer, the two membranes are glued to each other along their edges with the permeate spacer captured between them. The feed spacer remains unglued. A diagram of a cross-section of three wraps of a standard module is shown in FIG. 1. Module assemblies are wound up to a desired diameter and the outsides are sealed. In operation, multiple modules are placed in a tubular housing and fluid is pumped through them in series. The center tubes are plumbed together to allow removal of generated permeate.
Spiral wound membrane designs have been used successfully but can also foul with higher fouling feed streams. The fouling problem in standard spiral wound membranes is often due to the nature of the feed spacer that is required to be located through each of the feed channels. In addition, the presence of the feed spacer creates significant resistance to fluid flow. A typical feed spacer is a polymeric porous net-like material that the feed must be forced through in the longitudinal direction (i.e., the length) of the spiral wound membrane. Therefore, spiral wound membrane designs can also have fouling problems in the feed spacer and membrane and incur significant fluid dynamic problems due to resistance of the feed spacer. However, spiral wound designs are less expensive than alternatives for only less-fouling feed streams.
For the most fouling feed streams (for examples, solutions containing high levels of suspended solids or tend to form gels upon concentration) a tubular design membrane module has been designed. A tubular design provides the least amount of membrane surface area per module length, and is most expensive to manufacture due to labor intensive procedures for “potting” the tubular membranes within a module. Moreover, the inlet and outlet chambers associated with tubular designs are also most expensive. Therefore, there is a need in the art to replace the tubular design with a less expensive design and still be able to process highly fouling feed streams. The present invention was made to replace the tubular design with a spiral wound design for those feed streams that could not otherwise be processed (economically) in standard membrane modules having feed spacer designs.